1. Field of the Invention
The invention relates to a catalyst deterioration diagnosis system and method for an internal combustion engine.
2. Description of the Related Art
In general, exhaust systems for internal combustion engines are provided with various exhaust gas purification means, such as oxidation catalysts, NOx storage-reduction catalysts, particulate filters, etc. The components, such as hydrocarbon (HC), carbon monoxide (CO), NOx, particulate matter, contained in the exhaust gas that is discharged from an engine body are removed by such exhaust gas purification means. Of these, the oxidation catalyst is used to remove the unburned HC and CO in the exhaust gas that flows into the exhaust gas purification means.
Because of its property, the oxidation catalyst that oxidizes components contained in exhaust gas, which is catalyst noble metal, such as platinum (Pt), deteriorates and the oxidation capability thereof is reduced when the oxidation catalyst is exposed to high temperature and/or used for a long time. When the oxidation capability is reduced, the unburned HC and CO are less oxidized on the oxidation catalyst.
Thus, it is proposed to detect the degree of deterioration of the oxidation catalyst. For example, there is a catalyst deterioration diagnosis system in which the temperature of the oxidation catalyst is detected by a temperature sensor and that includes: an overheat temperature computing means that computes the overheat temperature that is the temperature of the oxidation catalyst that exceeds a catalyst deterioration determination temperature that is set in advance; an exceeding-time computing and adding-up means that adds up the exceeding time periods that are operating time weighed according to the increase in the detected overheat temperature; and a catalyst deterioration diagnosis means that determines the degree of deterioration of the oxidation catalyst based on the added-up exceeding time period (see Japanese Patent Application Publication No. 2005-307745 (JP-A-2005-307745), for example).
The longer the time period during which the temperature of the oxidation catalyst is equal to or higher than the catalyst deterioration determination temperature is and the higher the overheat temperature that is higher than the catalyst deterioration determination temperature is, the further the oxidation catalyst deteriorates. Thus, the catalyst deterioration diagnosis means determines the degree of deterioration of the oxidation catalyst based on such a time period and/or overheat temperature.
Meanwhile, the oxidation catalyst deteriorates due to not only the thermal deterioration caused by overheating of the oxidation catalyst but also other various causes, such as sulfur poisoning. However, in the catalyst deterioration diagnosis system as described in JP-A-2005-307745, the deterioration of the oxidation catalyst is determined based only on the thermal deterioration caused by overheating of the oxidation catalyst, and therefore, it is difficult to correctly determine the degree of deterioration of the oxidation catalyst when the oxidation catalyst deteriorates due to the causes other than overheating of the oxidation catalyst.